Hydroprocessing or hydrotreating reactions involve the application of hydrogen to a substrate, usually under elevated temperature and pressure, in the presence of a catalyst, with the aim of causing a physical or chemical change in the substrate. Most such hydroprocessing reactions occur in refinery operations where the substrate is a hydrocarbon feedstock.
Conventional hydroprocessing catalysts are generally in the form of a carrier of a refractory oxide material on which hydrogenation metals are deposited, the choice and amount of each component being determined by the end use. Refractory oxide materials usual in the art are amorphous or crystalline forms of alumina, silica and combinations thereof (though for some applications, materials such as titania may be used). These oxide materials can have some intrinsic catalytic activity but often only provide the support on which active metals compounds are held. The metals are generally base or noble metals from Group VIII and Group VIB of the Periodic Table which are deposited in oxidic form during manufacture; in the case of base metals, the oxides are then sulphided prior in order to use to enhance their activity.
Alternative catalyst forms have been proposed for use in the hydroprocessing of, for example, refinery streams. One such group of catalysts are termed ‘bulk metal catalysts’. Such catalysts are formed mainly from metal compounds, usually by co-precipitation techniques, and have no need for a catalyst carrier or support; see for example WO 00/42119 and U.S. Pat. No. 6,162,350. Both publications disclose bulk metal catalysts comprising Group VIII and Group VIb metals. U.S. Pat. No. 6,162,350 discloses that such catalysts may contain one or more of each metal type, and examples show NiMo, NiW and the most preferred NiMoW bulk metal catalysts.
The preparation of such trimetallic bulk catalyst particles comprising at least one metal at least partly in solid state form, is also described in WO 00/41810.
Further bulk metal-type catalysts and processes for their production are described in WO 2004/073859. The catalysts, termed ‘bulk metal oxide catalysts’, disclosed in this document contain a refractory oxide material, which is not used as a catalyst support. For example, the compositions described in WO 2004/073859 have the form (X)b(M)c(Z)d(O)e, wherein X represents at least one non-noble Group VIII metal; M represents one non-noble Group VIb metal; Z represents one or more elements selected from aluminium, silicon, magnesium, titanium, zirconium, boron, and zinc; one of b and c is the integer 1; and d and e and the other of b and c each are a number greater than 0 such that the molar ratio of b:c is in the range of from 0.5:1 to 5:1, the molar ratio of d:c is in the range of from 0.2:1 to 50:1, and the molar ratio of e:c is in the range of from 1:1 to 50:1.
Further examples of related catalyst compositions include an unsupported catalyst composition which comprises one or more Group VIb metals, one or more Group VIII metals, and a refractory oxide material which comprises 50 wt % or more titania, on oxide basis as described in WO 2004/073854, and an unsupported bulk metal oxide catalyst composition which comprises one or more Group VIb metals, one or more non-noble Group VIII metals, one or more zeolites, and, optionally, a refractory oxide material as disclosed in WO 2006/027359.
Hereinafter, for ease of understanding, the term ‘bulk metal catalyst’ will be used to refer to any bulk metal or bulk metal oxide catalyst.
In order to be used as active catalysts, most hydrotreating catalysts, including bulk metal catalysts, must be converted to their sulfidic, or sulfide, form (i.e. sulfided). Such activation may be carried out as part of the start-up of reaction processes using these catalysts. Such a start-up can be carried out while contacting the catalyst with the full-range feed which is to be treated by the catalyst.
However, the use of a full-range feed during start-up is usually less than ideal. For example, if the hydrotreating catalyst is part of a catalyst bed containing more than one type of catalyst, starting up the process with the hydrotreating catalyst in contact with the full range feed may lead to other catalysts in the catalyst bed being poisoned by contaminants (e.g. sulfur, nitrogen and oxygen containing species) that have not been removed by the non- or partially-activated hydrotreating catalyst.
Furthermore, if the product hydrocarbon stream, having passed through the catalyst bed, is collected without separation or removal of the fraction which passed over the hydrotreating catalyst before it was sufficiently sulfided, then this can lead to undesirable contaminants being present in said product hydrocarbon stream.
Thus, it is advantageous to start-up the reaction process using the catalyst with a feed which contains fewer contaminants (e.g. sulfur, nitrogen and oxygen containing species) than a full-range feed.
It is known in the art that when using a feed containing fewer contaminants, an amount of sulfur-containing species may need to be added to the feed during start-up in order to ensure that enough sulfur is present to allow successful sulfidation of the catalyst
It has been found by the present inventor that, even after the addition of sulfur-containing species, once bulk metal catalysts have been contacted with hydrocarbon feeds containing low levels of contaminants during start-up or sulfidation, a performance loss is observed. This observed loss of, e.g. hydrodesulfurization, activity cannot be restored by changing to a full range feed, or other heavier distillate streams, after the start-up is completed.
It would be advantageous to provide a start-up process, using hydrocarbon feeds containing low levels of contaminants, in which such a loss of performance does not take place.